The overall goal of the research is to determine the biological significance of the post-translational modification of tubulin catalyzed by the enzyme tubulin: tyrosine ligase (TTL). This enzyme adds tyrosine to the carboxyl terminus of the alpha-chain of tubulin in the presence of ATP, Mg ions and K ion. There is no evidence that tyrosinated tubulin assembles into microtubules differently from non-tyrosinated tubulin. However, the turnover of terminal tyrosine in cell culture is at least 100 times more rapid than the turnover of tubulin itself. Microtubules undergoing steady state metabolism must be present for tyrosine turnover to take place. Anti-mitotic drugs, e.g., colchicine and podophyllotoxin, as well as Li ion, at a concentration effective in the treatment of manic-depressive illness, inhibit both steady state metabolism and detyrosination. The enzyme that detyrosinates the tubulin dimers contained in microtubules is not TTL. The specific objectives of this research are: to isolate and characterize the enzymatic activity responsible for detyrosination; to determine the significance of the rapid increase in TTL activity and tyrosinable tubulin that occur during development; and to determine whether analogous reactions occur in invertebrate organisms.